When printings are successively performed with an electrophotographic image forming apparatus, an amount of charge on toner in a developer stored in a developing device can be changed greatly, resulting in deviation of a developing density. More specifically, a toner concentration in the developer stored in the developing device is kept within a predetermined range by supplying toner according to a drop in the toner concentration resulting from development. The amount of charge on the toner supplied into the developing device gradually increases as the toner is mixed and stirred with carrier particles in the developer; however, immediately after the toner has been supplied, the amount of charge on the toner is not sufficient. When such toner is supplied in a large amount, toner charge-to-mass ratio (Q/M) of the developer becomes relatively small, causing an amount of toner particles that are deposited on an electrostatic latent image, which is at a predetermined charge potential on a latent-image carrier, to increase. As a result, the developing density increases. Meanwhile, when images having a low image-area ratio are successively printed, a condition where only a small amount of toner is supplied to the developer continues long, causing a large part of toner in the developer to be retained while being stirred in the developing device for a long period of time. This makes the toner charge-to-mass ratio (Q/M) of the developer relatively large, causing an amount of toner particles that are deposited on an electrostatic latent image, which is at the predetermined potential on a latent-image carrier, to decrease. As a result, the developing density decreases. Such an increase or decrease in the developing density results in deviation of the developing density.
An image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2001-343827 is configured to reduce deviation of the developing density by performing a toner supply process and a deposition-amount stabilizing process discussed below. The toner supply process is performed by supplying toner to a developing device according to difference between a toner concentration in a developer in the developing device measured with a toner concentration sensor and a predetermined control target value so that the toner concentration is kept within a predetermined range. The deposition-amount stabilizing process is performed in parallel with the toner supply process as follows. During successive printings, each time a predetermined number of sheets are printed out, a test toner image is formed on a photosensitive element serving as a latent-image carrier and an optical sensor measures a toner amount per unit area on the test toner image. If the measured deposition amount is greater than a target deposition amount, or, put another way, if a developing density is higher than a target value, the control target value of the toner concentration in the developer is lowered to lower the toner concentration. This causes carrier particles to rub against individual toner particles more actively and hence increases the toner charge-to-mass ratio (Q/M), thereby lowering the developing density toward the target density. On the other hand, if the measured deposition amount is smaller than the target deposition amount, or, put another way, if the developing density is lower than the target value, the control target value of the toner concentration in the developer is increased to increase the toner concentration. This causes carrier particles to rub against individual toner particles less actively and hence decreases the toner charge-to-mass ratio (Q/M), thereby increasing the developing density toward the target density.
By performing the deposition-amount stabilizing process in this way, deviation of the developing density can be reduced. When a color image forming apparatus including a plurality of developing devices that perform developing with different color toners employs the deposition-amount stabilizing process discussed above to stabilize toner deposition amounts on a color-by-color basis, the image forming apparatus can stably produce printouts where colors do not vary widely among the printouts.
However, this approach is disadvantageous in that combined colors are continuously printed in color tones that slightly differ from those of desired, or target colors. More specifically, colors to be reproduced by a color image forming apparatus are roughly divided into primary colors and combined colors. A primary color is a color represented by only a single toner. For instance, in a configuration where four toners of yellow (Y), magenta (M), cyan (C), and black (K) toners are used, a color represented by only one of the Y, M, C, and K toners is a primary color. In contrast, a combined color is a color represented by using two or more different toners. A combined color is reproduced by overlaying a plurality of primary-color toner images on one another; however, the approach discussed above causes a combined color to be unfavorably printed in a color tone slightly different from that of a desired color. A mixture ratio of different color toners is adjusted according to area-coverage ratios of the primary-color toner images to be overlaid; however, the difference between a reproduced color tone and a desired color tone results from accumulation of various factors and it is difficult to clearly specify the cause.
Meanwhile, the present inventors develop a novel color image forming apparatus that performs a color-reproduction-accuracy increasing process discussed below rather than the deposition-amount stabilizing process discussed above. More specifically, a toner deposition amount on a toner image depends on not only a toner concentration of a developer but also other control parameters. The control parameters include a charge potential at a latent-image carrier, a latent-image writing intensity (when a photosensitive element is used, an intensity of writing light), and a developing bias voltage. For instance, when a setting value for a control parameter related to image forming with the Y toner, which is one of the primary colors, is changed, a Y-toner deposition amount on the Y-toner image changes. Accordingly, a color tone (for instance, a combination of L*, a*, and b* values in the L*a*b* colorimetric system) of a Y-toner image on a printout also changes. A Y-parameter/color-tone equation, which is an equation expressing a relationship between the setting value for the control parameter and a color tone of the Y-toner image, can be studied in advance by performing a test printing under a fixed environmental condition including the temperature and the humidity. Similarly, an M-parameter/color-tone equation, which is an equation expressing a relationship between a setting value for a control parameter related to image forming of an M-toner image and a color tone of the M-toner image, and a C-parameter/color-tone equation, which is an equation expressing a relationship between a setting value for a control parameter related to image forming of a C-toner image and a color tone of the C-toner image can be studied in advance. As discussed above, a combined-color toner image is formed by overlaying Y-, M-, and C-toner images on one another. A parameter/color-tone equation for any combined color can be established based on the Y-, M-, and C-parameter/color-tone equations and an area-coverage ratio of the Y-, M-, and C-toner images. Furthermore, it is possible to establish, for each of various control parameters, a parameter correcting equation for calculating a correction amount based on a difference between a result of color measurement performed on an actually-printed combined-color toner image and a desired color for reduction of the difference between a printed color and the desired color. To effectuate this, the Y-, M-, and C-parameter/color-tone equations mentioned above are established and stored in a control unit in advance. Each time a predetermined number of sheets are printed out, a suitable color measurement area (area where color varies narrowly) suitable for color measurement is searched for across an overall image to be printed out based on image information. Subsequently, when the image has actually been printed, a spectrometer performs color measurement on the suitable color-measurement area on a printout; thereafter, a parameter correcting equation that allows reduction in the difference between a color measurement result and a desired color is established based on the color measurement result and the Y-, M-, and C-parameter/color-tone equations. After correction amounts for the various control parameters have been determined by using the parameter correcting equations, the control parameters are corrected to increase color reproduction accuracy.
The inventors have fabricated a test product of a color image forming apparatus that performs such a color-reproduction-accuracy increasing process to carry out test printing and found that the combined color in the suitable color measurement area in a printed image has been successfully reproduced with high accuracy. Furthermore, regarding the combined color not only in the suitable color measurement area but also in other areas, a difference between a color tone of the combined color in the printout and that of a desired color has successfully been greatly reduced.
However, the test product configured as discussed above has a disadvantage below. When an image to be printed according to an instruction from a user has great variation in color tone, there can be a situation where the image has no suitable color measurement area suitable for color measurement and therefore the color-reproduction-accuracy increasing process cannot be performed. For a small amount of printing such as printing of several sheets, the situation where the color-reproduction-accuracy increasing process cannot be performed will not pose a serious problem. However, if the situation where the color-reproduction-accuracy increasing process cannot be performed is kept for successive printings of dozens of sheets, color tone of the printouts can be disturbed greatly. By forming a predetermined combined-color toner image for color measurement on recording paper to perform color measurement of the combined-color toner image rather than searching an image on a printout produced according to a request from a user for a suitable color measurement area to perform color measurement, the color-reproduction-accuracy increasing process can be certainly performed irrespective of a type of an image formed according to a request from a user. However, with this configuration, a test printout, on which a combined-color toner image for color measurement is formed, is produced in addition to printouts produced according to an instruction from a user. Accordingly, this configuration forces a user to sort out the test printout. Such a sorting operation is considerably burdensome and therefore it is substantially impracticable to employ the configuration in which the test printout on which a combined-color toner image is produced for color measurement.